Make a nested reference in one Model class Table - django

Question

I'd like to make a table with such a structure
1. urls
2. views
3. models
4. templates
4.1 html
4.2 format
4.3 test
html, format and test live in the same level with others but reference to 4. templates
I could start from the simple easy model as:
class Topic(models.Model):
"""A topic the user is learning about."""
title = models.CharField(max_length=200)
sub_title = models.CharField(max_length=200)
How to reference 'sub_title' to 'title'

Answer 1

This looks like a tree-like structure. A tree is basically a set of nodes (here topics) where a node can have at most one parent. There are no cycles in the parent relation (hence a node's grandparent can not be itself for example).
One can define such tree-like structure, by adding a reference to the parent, and that reference can be None (for top level nodes). So we can model this with:
class Topic(models.Model):
"""A topic the user is learning about."""
title = models.CharField(max_length=200)
parent = models.ForeignKey(
'Topic',
on_delete=models.SET_NULL,
null=True,
default=None,
)
Since your topics have an index number as well, this means that we can add an IntegerField with the index, and prefereably, the index and parent need to be unique together, such that we can not define two 4.2s:
class Topic(models.Model):
"""A topic the user is learning about."""
index = models.IntegerField()
title = models.CharField(max_length=200)
parent = models.ForeignKey(
'Topic',
on_delete=models.SET_NULL,
null=True,
default=None,
)
class Meta:
unique_together = (('parent', 'index'),)
ordering = ('index',)
So now we model the hierarchy with:
Topic.objects.create(index=1, title='urls')
Topic.objects.create(index=2, title='views')
Topic.objects.create(index=3, title='models')
templates = Topic.objects.create(index=4, title='templates')
Topic.objects.create(index=1, title='html', parent=templates)
Topic.objects.create(index=2, title='format', parent=templates)
tests = Topic.objects.create(index=3, title='tests', parent=templates)
Now we can obtain the parent of a topic, by calling .parent:
tests.parent # templates
We can also obtain an (ordered) set of subtopics with .topic_set:
templates.topic_set # queryset with [html, format, tests]